Mr Sarkozy, who also promised the parents of two teenagers whose deaths sparked the violence that they would learn "the full truth" about how their sons died, said the situation in some deprived neighbourhoods had been deteriorating "for 30 years" and had to be tackled firmly. More than 30 people were under arrest last night in the rundown northeastern district of Clichy-sous-Bois after some of the most violent clashes between riot police and mainly immigrant youths that the country has seen for some years.

Groups of youths have set cars and rubbish bins alight, hurled stones at police and fired at least one bullet in running battles that began on Thursday after Ziad, 17, and Banou, 15, were electrocuted in an electricity substation. On Sunday night a teargas grenade was fired into a mosque.

Visiting the nearby Seine-Saint-Denis police headquarters, Mr Sarkozy maintained his hardline stance, saying policing would be stepped up to ensure every resident of France's poor immigrant estates - where unemployment can be five times the national average - had "the security they have a right to". He said 17 companies of CRS riot police would be assigned permanently to difficult neighbourhoods, along with seven mobile police squads. Plainclothes agents will be sent on to some estates to "identify gang leaders, traffickers and big shots," he added, promising a "national plan" to deal with delinquency by the end of the year.

Opposition politicians, human rights groups and even some members of his own centre-right UMP party have accused Mr Sarkozy of being more interested in high-profile repression than long-term prevention. They are also upset at his use of words such as rabble, yobs and louts, which they say is likely to stoke tensions further. "This isn't how we resolve these problems," a former Socialist prime minister, Laurent Fabius, said on French radio. "We need to act at the same time on prevention, education, housing, jobs ... and not play the cowboy."

But Mr Sarkozy, citing statistics that show 30 police patrols are stoned and as many cars burned every night on France's low-income housing estates, is unrepentant. "There are some gangs and traffickers who are living off the underground economy, off drug trafficking, who seem to think these neighbourhoods are beyond the authorities' reach, " he said on television on Sunday. So far, France's voters seem to back him: he is by far the most popular politician and is seen as a leading presidential candidate in 2007.

Along with a friend, Metin, 21, who survived, Ziad and Banou were on their way home when they saw other youths being stopped and questioned by police. Apparently believing they were being pursued, they scrambled over a high barbed wire fence to get into the substation.

At the police station Mr Sarkozy met relatives of the two youths. He said that a full investigation was under way but on Sunday he had said that according to his information the youths were not being chased when they climbed the substation fence. The parents of the boys yesterday refused to meet the "incompetent" Mr Sarkozy and demanded to meet the prime minister, Dominique de Villepin.